After Upgrade to NC11 no internal sharing possible

I am running NC 10.0.4 with LDAP support without any problems. I upgraded to NC 11.0.2 on a test instance, and since then I am not able to create internal shares anymore, even after disabling LDAP support. Creating external (link) shares is still possible.
When I try to create an internal share and type in a non-existing user, the interface reports that the user does not exist. (correct behaviour). When I type in an existing user, nothing happens, even when I press Enter.

I already tried this without LDAP support (create an share from a local user to a local user) to exclude problems with the directory.

Nextcloud version: 11.0.2
Operating system and version: Ubuntu 16.04
Apache or nginx version: 2.4.18
PHP version: 7.0.15

Does no one else have such a problem with NC11?